Akihiro Okamoto

Structure-based functional identification of a novel heme-binding protein from Thermus thermophilus HB8.

The TT1485 gene from Thermus thermophilus HB8 encodes a hypothetical protein of unknown function with about 20 sequence homologs of bacterial or archaeal origin. Together they form a family of uncharacterized proteins, the cluster of orthologous group COG3253. Using a combination of amino acid sequence analysis, three-dimensional structural studies and biochemical assays, we identified TT1485 as a novel heme-binding protein. The crystal structure reveals that this protein is a pentamer and each monomer exhibits a β-barrel fold. TT1485 is structurally similar to muconolactone isomerase, but this provided no functional clues. Amino acid sequence analysis revealed remote homology to a heme enzyme, chlorite dismutase. Strikingly, amino acid residues that are highly conserved in the homologous hypothetical proteins and chlorite dismutase cluster around a deep cavity on the surface of each monomer. Molecular modeling shows that the cavity can accommodate a heme group with a strictly conserved His as a heme ligand. TT1485 reconstituted with iron protoporphyrin IX chloride gave a low chlorite dismutase activity, indicating that TT1485 catalyzes a reaction other than chlorite degradation. The presence of a possible Fe–His–Asp triad in the heme proximal site suggests that TT1485 functions as a novel heme peroxidase to detoxify hydrogen peroxide within the cell.

Reactions of dichlorocarbene with sterically protected 1-phosphaallene and 1,3-diphosphaallene

Dichlorocarbene reacted with sterically protected 1-phosphaallene and 1,3-diphosphaallene to give a methylene-phosphirane and a methylene-diphosphirane, respectively; the former phosphirane was converted to 1-phosphabutatriene with butyllithium and the structure of the latter diphosphirane was determined by X-ray crystallography.